Xll INTRODUCTION. 



cistern which will not reward a search, and furnish specimens 

 of the tribe. 



The indestructible nature of their epiderm has also served to 

 perpetuate the presence of these forms in numerous localities, 

 from which their living representatives have long since dis- 

 appeared. Districts recovered from the sea, in the present or 

 other periods of the earth's history, frequently contain myriads 

 of such exuviae forming strata of considerable thickness. Such 

 deposits have been found in Bohemia, in the neighbourhood of 

 Berlin, in various districts in Italy, and in several of the American 

 States. The city of Richmond in Virginia is said to be built 

 upon a stratum of Diatomaceous remains 18 feet in thickness, 

 and extensive tracts in the Arctic Regions have been found 

 covered with similar relics of a former vegetation. 



Nor are we without examples, though on a less extensive 

 scale, in our own country. The ancient site of a mountain lake 

 in the neighbourhood of Dolgelly, localities of a somewhat similar 

 kind near Lough Island-Reavey, in Down, and Lough Mourne, 

 in Antrim, have furnished large supplies of some of the forms 

 described in the present work. Several deposits of a like kind 

 have been met with in Scotland, and have also contributed to en- 

 rich the present volumes. The extreme minuteness of the organ- 

 isms which have furnished such remains, and the hardness of their 

 material, have rendered the substance formed by their aggrega- 

 tion, a useful agent in the mechanical arts, in which it has been 

 employed to confer a polish upon hard surfaces. It is from this 

 circumstance that the material known as Tripoli derives its value 

 as a polisher of metals ; and the Dolgelly deposit has to some 

 extent been employed for a similar purpose. 



One of the most singular instances of the preservation of 

 Diatomaceous forms occm-s in regard to Guano, so largely im- 

 ported as a manure from Peru and Africa. This material is 

 found to contain a large number of the siliceous coverings of 

 these minute organisms, which have been swallowed by the ma- 

 rine birds frequenting the spots from which the Guano is pro- 

 cured, have survived the process of digestion to which they 

 were at first submitted, and the ages during which they have 

 been imbedded in decomposed or decomposing matter, and now 



